The estrogen receptor (ER) is a member of the nuclear hormone receptor family and functions as a ligand-activated transcription factor involved with the up and down regulation of gene expression. The natural hormone for the estrogen receptor is B17-estradiol (E2) and closely related metabolites. Binding of estradiol to the estrogen receptor causes a dimerization of the receptor and the dimer in turn binds to estrogen response elements (ERE's) on DNA. The ERDNA complex recruits other transcription factors responsible for the transcription of DNA downstream from the ERE into mRNA which is eventually translated into protein. Alternatively the interaction of ER with DNA may be indirect through the intermediacy of other transcription factors, most notably fos and jun. Since the expression of a large number of genes is regulated by the estrogen receptor and since the estrogen receptor is expressed in many cell types, modulation of the estrogen receptor through binding of either natural hormones or synthetic ER ligands can have profound effects on the physiology and pathophysiology of the organism.
A variety of diseases have their aetiology and/or pathology mediated by the ER. Collectively these diseases are called estrogen-dependent diseases. Estrogens are critical for sexual development in females. In addition, estrogens play an important role in maintaining bone density, regulation of blood lipid levels, and appear to have neuroprotective effects. Consequently decreased estrogen production in post-menopausal women is associated with a number of diseases such as osteoporosis, atherosclerosis, depression and cognitive disorders. Conversely certain types of proliferative diseases such as breast and uterine cancer and endometriosis are stimulated by estrogens and therefore antiestrogens (i.e. estrogen antagonists) have utility in the prevention and treatment of these types of disorders.
There are two different forms of the estrogen receptor, usually referred to as α and β, each encoded by a separate gene (ESR1 and ESR2, respectively).
Both ERs are widely expressed in different tissue types, however there are some notable differences in their expression patterns. The ERα is found in endometrium, breast cancer cells, ovarian stroma cells, and the hypothalamus. In males, ERα protein is found in the epithelium of the efferent ducts. The expression of the ERβ protein has been documented in kidney, brain, bone, heart, lungs, intestinal mucosa, prostate, and endothelial cells. Development therefore of selective ligands may therefore preserve the beneficial aspects of estrogen.
Breast cancer is the most common malignancy to affect women and worldwide, the incidence of the disease is increasing. Estrogens, in particular, act as endocrine growth factors for at least one-third of breast cancers, and depriving the tumour of this stimulus is a recognised therapy for advanced disease In premenopausal women, this is achieved by the ablation of ovarian function through surgical, radiotherapeutic, or medical means and, in postmenopausal women, by the use of aromatase inhibitors.
An alternative approach to estrogen withdrawal is to antagonise estrogen with antiestrogens. These are drugs that bind to and compete for estrogen receptors (ER) present in estrogen-responsive tissue. Conventional nonsteroidal antiestrogens, such as tamoxifen, compete efficiently for ER binding but their effectiveness is often limited by the partial agonism they display, which results in an incomplete blockade of estrogen-mediated activity. A specific or “pure” antiestrogen with high affinity for ER and without any agonist effects, may have advantages over conventional nonsteroidal anti estrogens in the treatment of estrogen-dependent disease. Fulvestrant is the first of a new class of potent pure anti estrogens and is completely free of the partial agonist, estrogen-like activity, associated with currently available antiestrogens like tamoxifen.
It would be desirable to investigate other approaches to antagonise the ER receptor.
One approach would be to develop selective ER down regulators or degraders resulting in the reduction of ER expression at either the transcript or protein level.
Several methods are available for the manipulation of protein levels, including proteolysis targeting chimeric molecules (PROTACs) which contain a ligand that recognizes the target protein linked to a ligand that binds to a specific E3 ubiquitin ligase. It would be desirable to have a small molecule which can simultaneously bind ER and an E3 ubiquitin ligase and which promotes ubiquitination of ER and leads to degradation of ER by the Proteosome. One suitable E3 ubiquitin ligase is the von Hippel-Lindau tumour suppressor (VHL).
The present inventors have identified compounds which are capable of inhibiting estrogen receptor function including compounds which degrade the estrogen receptor.